1. Field of the Invention
The present invention relates to a surface emitting laser device, a surface emitting laser array, a method of producing the surface emitting laser device, a surface emitting laser module, an electrophotographic system, an optical communication system, and an optical interconnection system.
2. Description of the Related Art
Because a surface emitting laser (for example, a surface emitting semiconductor laser) has a small-volume active layer and is capable of operations at a low threshold current and high speed modulation compared to an edge-emitting laser, it is attracting attention as a light source in a LAN (Local Area Network) or the like. In addition, since the surface emitting laser emits a laser beam in a direction perpendicular to a substrate thereof, it is easy to construct a two-dimensional array structure; thus, it is expected that the surface emitting laser is to be used as a light source for a parallel optical interconnection, or a writing light source array in a high-resolution electrophotographic system.
Because the surface emitting laser has a small-volume active layer, it can be operated at a low threshold compared to the edge-emitting laser. On the other hand, because of the small volume of the active layer, it is difficult for the surface emitting laser to obtain high output.
In addition, in applications such as optical fiber communications, a light source for an electrophotographic system, or others, laser oscillation in the single fundamental transverse mode is desirable, because such a mode results in a circular outgoing laser beam. However, it is difficult to maintain the oscillation in the single fundamental transverse mode even at high output.
Presently, there are two typical structures of the surface emitting laser, one is a so-called “selective oxidation structure”, the other is a so-called “ion (hydrogen) implantation structure”.
In a selective oxidation surface emitting laser device, oscillation in the single fundamental transverse mode is obtained by a well-known method of setting one side or the diameter of a non-oxidation region of a current-confinement structure, which is formed by selective oxidation of a semiconductor layer including aluminum, to be three or four times the oscillation wavelength. Since the selectively oxidized region has a refractive index lower than that of the semiconductor layer nearby, an effective refractive index waveguide structure is formed, and when one side or the diameter of the non-oxidation region is set as above, the refractive index waveguide structure satisfies the cut-off condition of high order transverse modes.
However, in this case, the relevant side or the diameter of the non-oxidation region is actually as small as 3 or a few more microns, so that the resistance of the current-confinement structure increases, and thereby the driving voltage of the laser device becomes high.
When the current injection region is small, the oscillation region becomes small. Further, heat generated in the laser device rises due to an increased resistance; as a result, it becomes difficult to obtain high output.
When actually driving the laser device, it is known that it is difficult to maintain the oscillation in the single fundamental transverse mode in a current highly-injected region because of the refractive index changes caused by heat generation in a current passage and plasma effect of injected carrier, gain saturation of the single fundamental transverse mode due to a spatial hole burning effect.
On the other hand, in a hydrogen ion implantation surface emitting laser device, in which a high resistance region formed by hydrogen ion implantation is provided inside the laser device for current confinement, the refractive index of the hydrogen ion implantation region barely changes, and transverse optical confinement is performed due to a difference of the refractive index caused by a temperature change during current conduction. Because this change of the refractive index is very small compared to that in the transverse optical confinement in the selective oxidation surface emitting laser device, it is possible to obtain operations in the single fundamental transverse mode even at a relatively large diameter of the current confinement. However, as is known, this kind device suffers from oscillation delay, or poor stability of the transverse mode.
As described above, it is difficult to obtain stable oscillation in the single fundamental transverse mode at high output with either the selective oxidation surface emitting laser device or the hydrogen ion implantation surface emitting laser device.
For example, Japanese Laid Open Patent Application No. 2003-115634 (hereinafter, referred to as “reference 1”), Japanese Laid Open Patent Application No. 2002-208755 (hereinafter, referred to as “reference 2”), and Japanese Laid Open Patent Application No. 2004-23087 (hereinafter, referred to as “reference 3”) propose methods for solving the above problems to realize operations at high output in the single fundamental transverse mode particularly in the selective oxidation surface emitting laser device.
Specifically, in reference 1, focusing on the fact that the high order transverse modes have a mode distribution near the current injection region of a device, in a semiconductor layer forming a distributed Bragg reflector (DBR) on the surface of the laser device, a region corresponding to the high order transverse modes is etched, and a relief structure is provided on the surface of the laser device. Because of a change of phase conditions for multiple-reflection (that is, inversion of the phase conditions), reflectivity declines in the region where the distributed Bragg reflector is processed by etching, and it is possible to increase loss of the high order transverse modes. In other words, while a mode distribution exists in a region corresponding to the current injection region, without affecting the single fundamental transverse mode, it is attempted to obtain oscillation in the single fundamental transverse mode even at high output by adding the loss of the high order transverse modes.
In reference 2 and reference 3, based on an idea similar to reference 1, it is attempted to decrease the reflectivity of a distributed Bragg reflector near the current injection region by alloying a semiconductor contact layer on the surface of a device with a metal, and to obtain oscillation in the single fundamental transverse mode even at high output.
The above reference 1, reference 2, and reference 3 disclose methods of adjusting the reflectivity on the surface of the distributed Bragg reflector. However, because the distributed Bragg reflector utilizes multiple-reflections on interfaces of multiple layers, the light intensity is attenuated gradually towards the surface layer; thus, by arranging a reflectivity adjusting structure on the surface of the laser device, it is difficult to add a large mirror loss.
For this reason, in the related art, it is difficult to selectively add a sufficiently large mirror loss, and when the current is injected strongly, oscillation in the high order transverse modes is started. In other words, the high order transverse mode oscillation cannot be sufficiently suppressed.